1. Field of the Invention
This invention relates to a magnetic toner for developing electrostatic images to form toner images in image forming processes such as electrophotography. It also relates to a process for producing the magnetic toner, an image forming method useing the magnetic toner and a process cartridge having the magnetic toner.
2. Related Background Art
In recent years, image forming apparatus employing electrophotographic techniques, such as copying machines and laser beam printers, have come to function in a variety of ways and are sought to make images have much higher minuteness and higher image quality.
Accordingly, there is a tendency to employ magnetic toner particles having finer particle diameters than ever. When the magnetic toner particles are made to have smaller particle diameters, images can be made to have higher minuteness. On the other hand, the phenomenon of fog caused by adhesion of magnetic toner to non-image areas tends to occur especially in an environment of low temperature and low humidity. In an environment of high temperature and high humidity, images tend to be formed in a low density when copies or prints are taken firstly in the morning. Moreover, nowadays, taking account of environmental problems, it has come to employ a method of charging photosensitive members by means of contact charging members without using ozone-causative corona charging assemblies.
In this instance, however, fine particles of magnetic toner particles that are not sufficiently removed by cleaning with a cleaning member may adhere to the contact charging member (hereinafter "charging-roller contamination") in the environment of low temperature and low humidity to cause faulty charging, which may further cause faulty images. In the environment of high temperature and high humidity, the above fine particles tend to adhere to the surface of the photosensitive drum which is an electrostatic latent image bearing member, when they are pressed against it by the contact charging member (this phenomenon is herein called "melt-adhesion to drum").
It has been ascertained that such fine particles are comprised chiefly of silica fine powder and/or magnetic fine power, the former being used as a fluidity improver and the latter being a material constituting the magnetic toner particles. Moreover, in the case of the magnetic toner having much finer particle diameter than ever as stated above, the magnetic fine powder tends to more adhere to the contact charging member and photosensitive drum.
A method is conventionally known in which the particle surfaces of the magnetic fine powder are previously treated with an organic matter in order to improve the close contact of binder resin to magnetic fine powder. This, however, tends to cause faulty coating (a blotch phenomenon) when a magnetic toner layer is applied onto a toner carrying member in the environment of low temperature and low humidity. The surface treatment of the magnetic fine powder may also result in a higher production cost.
As means for solving the above problems, it is long-awaited to propose a novel toner in which the state of presence of magnetic fine powder on the surfaces of magnetic toner particles has been controlled, and to propose a novel process for producing such a toner.
It is difficult to uniformly disperse all materials such as binder resin, magnetic fine powder and wax in a kneaded product. For example, the kneading conditions taking account of the wettability of magnetic fine powder by binder resin and the kneading conditions taking account of the dispersibility of binder resin in wax are incompatible with each other.
In Japanese Patent Application Laid-Open No. 8-123083, a toner production process is proposed which specifies temperature conditions required when the materials are melt-kneaded by means of a screw extruder having a feed screw zone and a kneading zone. Examples set out in this publication disclose a process for producing a magnetic toner having a volume-average particle diameter (d50) of from 7.15 to 7.23 .mu.m. Even in this production process, as the magnetic toner comes to have a smaller average particle diameter, the magnetic fine particles tend to become liberated from the magnetic toner particle surfaces to highly tend to result in an increase in the number of free magnetic fine particles. Also, in this production process, the temperature in the extruder is set lower on the outlet side of the kneaded product, which is required for compulsorily cooling the kneaded product heated in the extruder. It is commonly difficult to control such temperature, which requires so great a load that the process is hard to control in actual production.